Most commercially available processes for producing STP produce undesirable fines which cause dusting and difficulties in handling the STP. The fines are typically removed from the larger granules by screening or other methods of separation. The fines are typically not pure STP, and can have up to 20 weight % of other phosphate compounds, such as metaphosphates, pyrophosphates and orthophosphates. The present invention provides a method of utilizing the fines by reprocessing them to produce medium density granular STP.
U.S. Pat. No. 3,160,472 discloses the agglomeration of STP fines by calcining the fines in the presence of at least 10% of blends of certain sodium orthophosphate salts. U.S. Pat. No. 3,761,573 discloses the spraying of fines with an sodium orthophosphate solution to form particles, drying the particles, screening to recover the desired fractions and annealing the recovered particles. U.S. Pat. No. 3,932,590 discloses the spraying of STP fines with a solution of sodium orthophosphates and calcining to produce granular STP. U.S. Pat. No. 4,134,963 discloses the agglomeration of STP fines by spraying with water, then calcining. U.S. Pat. No. 4,251,498 discloses the compaction, milling, moisturizing and calcining of STP fines to agglomerate the fines and produce a granular product. U.S. Pat. No. 3,650,685 discloses the spray drying of an aqueous orthophosphate salt with recycle of fines which are hydrated to a loss on ignition or LOI of between about 18% and 25% by weight and being mixed with the course material from the spray drier. The conditioned feed is converted to granular, low density sodium phosphates at a temperature in the range of from 200.degree. C. to about 500.degree. C. in a fluidized bed. Dense granules of STP are converted to medium density granular STP by means of first moisturizing the STP with water to produce a material having between about 10% and about 30% by weight moisture. The moisturized material is calcined to a temperature within the range of from about 300.degree. C. to about 600.degree. C. to produce a granular STP having low frangibility (friability) and high absorptivity. According to U.S. Pat. No. 4,556,525, spray dried sodium phosphates are agglomerated in a blender as a powder and sprayed with water to 8.4% by weight. The agglomerates are then calcined to provide a medium density polyphosphate.
None of the above patents disclose the present invention of agglomerating STP fines by first compacting the fines in admixture with dried flakes of an sodium orthophosphate, then calcining the compacted material to provide a granular product having a medium range bulk density.